This invention relates to the production of one-side electroplated steel sheet or coil plate (hereunder collectively referred to as steel sheet) with the other side, i.e. non-electroplating side having a high affinity for chemical converting treatment (e.g. phosphating).
The term "one-side electroplated steel sheet" means the steel sheet having one side electroplated and the other opposite side free from electroplating. The other opposite side free from electroplating is hereunder called "non-electroplating side (or surface)" merely for the purpose of distinguishing it from the electroplated side, though even the "non-electroplating side" of steel sheet is slightly electroplated during plating according to this invention.
Recently, one-side electroplated steel sheet has been extensively used as corrosion-resistant material in the automotive industry. In particular one-side electroplated sheet of high strength steel is advantageously used to produce light weight cars with improved mileage economy.
The electroplated steel sheet used for making automobiles etc. is, after being shaped, usually subjected to electropainting, seal-coating and then top-coating, and as a pre-treatment to such painting a chemical conversion treatment such as phosphating is usually carried out so as to improve the adhesion of a paint film to the substrate and its corrosion performance.
As is well known in the art, it is necessary to make the surface to be treated clean so as to promote the formation of nuclei for crystal growth on the surface of the substrate during chemical conversion treatment. However, in the case of electroplated steel sheet since the electroplating is carried out in an acidic bath, the non-electroplating surface of the sheet is gradually corroded by an acid in the plating bath during the one-sided plating, resulting in the formation of corrosion products on the surface of the non-plating side. The thus formed corrosion products prevent the formation of crystal nuclei in the course of the chemical conversion treatment. Therefore, in order to provide the non-plating surface with satisfactory chemical conversion properties, it is necessary either to prevent the formation of the corrosion products during electroplating or to remove the once formed corrosion products prior to the chemical conversion treatment.
In the prior art, in order to cope with such problems as in the above the corrosion products has generally been removed after plating. Namely, since the corrosion products are so firmly adhered to the substrate surface that it is difficult to thoroughly remove them merely by means of washing, either one of the following two methods has been used; one is to mechanically grind the non-electroplating surface as a post-treatment to electroplating so as to remove yellow discolored spots as well as the corrosion products; and the other is to subject the non-electroplating surface to electrolytic pickling in an acidic bath after the electroplating so as to remove the corrosion products from the surface, and then plating a specific metal such as nickel in a very small amount with a uniform distribution thereof throughout the surface, resulting in an improvement in the affinity for surface treatment.
However, according to the surface grinding type method multiple stages of grinding are required to thoroughly remove the corrosion products or foreign materials from the substrate surface. Thus, this method is very expensive. Furthermore, since it is difficult to uniformly remove the corrosion products so long as mechanical grinding is employed, unevenness in grinding is unavoidable, resulting in a considerable variation in phosphate crystal size from place to place in the chemical conversion layer. Therefore, the unevenness in grinding brings about a degradation in the affinity for chemical conversion treatment.
In addition, a substantial amount of scratch flaws remain on the surface of the sheet after grinding, and they adversely affect paint film appearance and corrosion resistance of the finishing coating to an extent depending on the depth and density of such grinding scratches. The formation of yellowish rust comprised of hydroxides is also inevitable due to the presence of water remaining on the surface of the steel sheet when it is washed with water after grinding. The formation of such yellowish rust degradates the affinity for chemical conversion of the resulting steel sheet.
According to another method in which a specific metal is electroplated onto the non-electroplating surface of the sheet, it is necessary to provide the plating line with additional costly means such as a series of radial cells in order to deposit the specific metal onto only the non-electroplating surface, making the process expensive. In addition, the electrolytic pickling and the subsequent plating of such a specific metal are carried out in an acidic bath, and the other side of the sheet on which the finally desired objective electroplated layer has already been provided is gradually dissolved or discoloured during the electrolysis and plating. Such dissolution and discolouration possibly degrade the commercial value of the final electroplated product.